This invention relates to electrochemical reactors for conducting reduction and oxidation reactions in respective positive and negative electrolyte solutions, without gas evolution at the electrodes. More specifically the invention relates to a membrane-separated, multicell electrochemical reactor for implementing a redox flow battery system.
Redox (reduction/oxidation) flow battery systems are increasingly attracting interest as efficient energy conversion systems. Among redox couple candidates, the all vanadium redox system is often preferred.
Structurally, electrochemical reactors that have been proposed for redox flow battery systems have been derived from the electrochemical reactor structures developed for general electrolysis processes, the only adaptation having concerned the materials employed as electrodes.
Generally, electrochemical reactors used as redox batteries are composed of a stack of electrode elements separated by ion exchange membranes, defining a positive electrolytic solution (“positive electrolyte”) flow chamber on one side of each membrane, and a negative electrolytic solution (“negative electrolyte”) flow chamber on the opposite side thereof. The stack of component elements is assembled together in a filter-pass arrangement between two end electrode elements.
U.S. Pat. No. 4,886,586 discloses a frame assembly for ion exchange membrane consisting of two frames holding the membrane therebetween. The assembly is realizably fastened by means of separate screws screwed through a counterflange, the membrane and the inner flange of the frame.
Commonly, the elements have a frame provided with coordinated through holes forming inlet and outlet manifolds for the two electrolytes that are circulated in a parallel mode through the positive electrolyte flow chambers and the negative electrolyte flow chambers, respectively. Traditionally the elements are mounted and operated in a vertical position.
A redox system requires nonnegligible electrolyte flow rates through the flow chambers of the reactor in order to maintain optimal half-cell reactions conditions at the electrodes.
A membrane-separated multicell electrochemical reactor for half-cell reduction and oxidation reactions in respective positive and negative electrolytes, without gas evolution, may even have an architecture that makes it easier to assemble by allowing stacking fully pre-assembled elements horizontally, one on top of the other, and which is suitable for operation in the same horizontal orientation of the bipolar elements.
The multicell assembly may be constituted by alternately stacking two types of pre-assembled elements, one may be a bipolar electrode subassembly and the other a membrane subassembly.
The alternate stack of elements may be piled over a bottom end electrode element, and the stack may be terminated by placing, over the last membrane element, a top end electrode element. The two end elements eventually compress the stack upon tightening a plurality of tie rods, commonly arranged around the perimeter of the stacked elements, according to the common practice of tightening a filter-press stack in a hydraulically sealed manner, by virtue of gaskets operatively installed between the coupling faces of the frames of the stacked elements.
Each electrode element and each ion exchange membrane separator element may include a substantially similar rectangular frame piece, usually made of an electrically nonconductive and chemically resistant material, typically of molded plastic material, having through holes and recesses in coordinated locations disposed along two opposite sides of the rectangular frame forming, upon completion of the assembling, ducts for the separate circulation of the negative electrolyte and of the positive electrolyte through all the negative electrolyte flow chambers and all positive electrolyte flow chambers, respectively.
According to a cascade flow mode, the negative electrolyte enters along a first side of a negative electrolyte flow chamber, flows through the chamber toward the opposite or second side thereof, exits the chamber, flows through the coordinated holes through the frame holding the electrode and through the frame holding the next membrane separator, reaching the level of the next negative electrolyte flow chamber and enters it from the same second side through which it exited from the previous negative electrolyte flow chamber and exits this next negative electrolyte flow chamber from the same first side it entered the previous negative electrolyte flow chamber, to flow through coordinated holes through the next pair of frames to the level of the next negative electrolyte flow chamber and so forth.
The same flow path is arranged also for the positive electrolyte, either in a “countercurrent” or in an “equicurrent” mode through the battery.
According to such an architecture, the electrochemical reactor does not have inlet and outlet manifolds for the two electrolytes. On the contrary, the electrolytes flow through the respective flow chambers in a zigzag path, that is essentially in hydraulic series or cascade mode instead of in hydraulic parallel mode.
In this way, by-pass current may only be “driven” by a voltage difference of about one-cell voltage, and becomes practically negligible and, above all, it does not cause any corrosion on conductive parts.
Irrespective of the particular stack architecture, and most preferably, each electrode consists of a porous fabric or mat, commonly of carbon fibers, in electrical continuity with a similar electrode structure on the opposite face of a conductive current collecting plate for providing substantially three-dimensional electrode structures having a large active area, that in many cases may occupy almost entirely the relative electrolyte flow chamber.
This arrangement, dictated by the need to enhance the rate of the half-cell reaction that can be supported at the electrode, contrasts with the need of minimizing the power absorbed by the motors that drive the electrolytes pumps in order to flow the solutions through the plurality of respective flow chambers at an adequate flow rate.
As may be readily recognized, this problem is aggravated when passing from a traditional parallel flow of the electrolyte through all the respective flow chambers from a common inlet manifold to common outlet manifold, to a cascade flow from one chamber to the next, starting from one end to the opposite end of the multicell stack.
Although the cascade flow mode is extremely effective in eliminating any corrosion problems due to by-pass currents, it necessarily implies an augmented pressure drop in flowing the two electrolytes through the battery.